docs: block: remove queue-sysfs.rst

   kyber-iosched

   null_blk

   pr

   queue-sysfs

   request

   stat

   switching-sched

=================

Queue sysfs files

=================

This text file will detail the queue files that are located in the sysfs tree

for each block device. Note that stacked devices typically do not export

any settings, since their queue merely functions as a remapping target.

These files are the ones found in the /sys/block/xxx/queue/ directory.

Files denoted with a RO postfix are readonly and the RW postfix means

read-write.

add_random (RW)

---------------

This file allows to turn off the disk entropy contribution. Default

value of this file is '1'(on).

chunk_sectors (RO)

------------------

This has different meaning depending on the type of the block device.

For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors

of the RAID volume stripe segment. For a zoned block device, either host-aware

or host-managed, chunk_sectors indicates the size in 512B sectors of the zones

of the device, with the eventual exception of the last zone of the device which

may be smaller.

dax (RO)

--------

This file indicates whether the device supports Direct Access (DAX),

used by CPU-addressable storage to bypass the pagecache.  It shows '1'

if true, '0' if not.

discard_granularity (RO)

------------------------

This shows the size of internal allocation of the device in bytes, if

reported by the device. A value of '0' means device does not support

the discard functionality.

discard_max_hw_bytes (RO)

-------------------------

Devices that support discard functionality may have internal limits on

the number of bytes that can be trimmed or unmapped in a single operation.

The `discard_max_hw_bytes` parameter is set by the device driver to the

maximum number of bytes that can be discarded in a single operation.

Discard requests issued to the device must not exceed this limit.

A `discard_max_hw_bytes` value of 0 means that the device does not support

discard functionality.

discard_max_bytes (RW)

----------------------

While discard_max_hw_bytes is the hardware limit for the device, this

setting is the software limit. Some devices exhibit large latencies when

large discards are issued, setting this value lower will make Linux issue

smaller discards and potentially help reduce latencies induced by large

discard operations.

discard_zeroes_data (RO)

------------------------

Obsolete. Always zero.

fua (RO)

--------

Whether or not the block driver supports the FUA flag for write requests.

FUA stands for Force Unit Access. If the FUA flag is set that means that

write requests must bypass the volatile cache of the storage device.

hw_sector_size (RO)

-------------------

This is the hardware sector size of the device, in bytes.

io_poll (RW)

------------

When read, this file shows whether polling is enabled (1) or disabled

(0).  Writing '0' to this file will disable polling for this device.

Writing any non-zero value will enable this feature.

io_poll_delay (RW)

------------------

If polling is enabled, this controls what kind of polling will be

performed. It defaults to -1, which is classic polling. In this mode,

the CPU will repeatedly ask for completions without giving up any time.

If set to 0, a hybrid polling mode is used, where the kernel will attempt

to make an educated guess at when the IO will complete. Based on this

guess, the kernel will put the process issuing IO to sleep for an amount

of time, before entering a classic poll loop. This mode might be a

little slower than pure classic polling, but it will be more efficient.

If set to a value larger than 0, the kernel will put the process issuing

IO to sleep for this amount of microseconds before entering classic

polling.

io_timeout (RW)

---------------

io_timeout is the request timeout in milliseconds. If a request does not

complete in this time then the block driver timeout handler is invoked.

That timeout handler can decide to retry the request, to fail it or to start

a device recovery strategy.

iostats (RW)

-------------

This file is used to control (on/off) the iostats accounting of the

disk.

logical_block_size (RO)

-----------------------

This is the logical block size of the device, in bytes.

max_discard_segments (RO)

-------------------------

The maximum number of DMA scatter/gather entries in a discard request.

max_hw_sectors_kb (RO)

----------------------

This is the maximum number of kilobytes supported in a single data transfer.

max_integrity_segments (RO)

---------------------------

Maximum number of elements in a DMA scatter/gather list with integrity

data that will be submitted by the block layer core to the associated

block driver.

max_active_zones (RO)

---------------------

For zoned block devices (zoned attribute indicating "host-managed" or

"host-aware"), the sum of zones belonging to any of the zone states:

EXPLICIT OPEN, IMPLICIT OPEN or CLOSED, is limited by this value.

If this value is 0, there is no limit.

If the host attempts to exceed this limit, the driver should report this error

with BLK_STS_ZONE_ACTIVE_RESOURCE, which user space may see as the EOVERFLOW

errno.

max_open_zones (RO)

-------------------

For zoned block devices (zoned attribute indicating "host-managed" or

"host-aware"), the sum of zones belonging to any of the zone states:

EXPLICIT OPEN or IMPLICIT OPEN, is limited by this value.

If this value is 0, there is no limit.

If the host attempts to exceed this limit, the driver should report this error

with BLK_STS_ZONE_OPEN_RESOURCE, which user space may see as the ETOOMANYREFS

errno.

max_sectors_kb (RW)

-------------------

This is the maximum number of kilobytes that the block layer will allow

for a filesystem request. Must be smaller than or equal to the maximum

size allowed by the hardware.

max_segments (RO)

-----------------

Maximum number of elements in a DMA scatter/gather list that is submitted

to the associated block driver.

max_segment_size (RO)

---------------------

Maximum size in bytes of a single element in a DMA scatter/gather list.

minimum_io_size (RO)

--------------------

This is the smallest preferred IO size reported by the device.

nomerges (RW)

-------------

This enables the user to disable the lookup logic involved with IO

merging requests in the block layer. By default (0) all merges are

enabled. When set to 1 only simple one-hit merges will be tried. When

set to 2 no merge algorithms will be tried (including one-hit or more

complex tree/hash lookups).

nr_requests (RW)

----------------

This controls how many requests may be allocated in the block layer for

read or write requests. Note that the total allocated number may be twice

this amount, since it applies only to reads or writes (not the accumulated

sum).

To avoid priority inversion through request starvation, a request

queue maintains a separate request pool per each cgroup when

CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such

per-block-cgroup request pool.  IOW, if there are N block cgroups,

each request queue may have up to N request pools, each independently

regulated by nr_requests.

nr_zones (RO)

-------------

For zoned block devices (zoned attribute indicating "host-managed" or

"host-aware"), this indicates the total number of zones of the device.

This is always 0 for regular block devices.

optimal_io_size (RO)

--------------------

This is the optimal IO size reported by the device.

physical_block_size (RO)

------------------------

This is the physical block size of device, in bytes.

read_ahead_kb (RW)

------------------

Maximum number of kilobytes to read-ahead for filesystems on this block

device.

rotational (RW)

---------------

This file is used to stat if the device is of rotational type or

non-rotational type.

rq_affinity (RW)

----------------

If this option is '1', the block layer will migrate request completions to the

cpu "group" that originally submitted the request. For some workloads this

provides a significant reduction in CPU cycles due to caching effects.

For storage configurations that need to maximize distribution of completion

processing setting this option to '2' forces the completion to run on the

requesting cpu (bypassing the "group" aggregation logic).

scheduler (RW)

--------------

When read, this file will display the current and available IO schedulers

for this block device. The currently active IO scheduler will be enclosed

in [] brackets. Writing an IO scheduler name to this file will switch

control of this block device to that new IO scheduler. Note that writing

an IO scheduler name to this file will attempt to load that IO scheduler

module, if it isn't already present in the system.

write_cache (RW)

----------------

When read, this file will display whether the device has write back

caching enabled or not. It will return "write back" for the former

case, and "write through" for the latter. Writing to this file can

change the kernels view of the device, but it doesn't alter the

device state. This means that it might not be safe to toggle the

setting from "write back" to "write through", since that will also

eliminate cache flushes issued by the kernel.

write_same_max_bytes (RO)

-------------------------

This is the number of bytes the device can write in a single write-same

command.  A value of '0' means write-same is not supported by this

device.

wbt_lat_usec (RW)

-----------------

If the device is registered for writeback throttling, then this file shows

the target minimum read latency. If this latency is exceeded in a given

window of time (see wb_window_usec), then the writeback throttling will start

scaling back writes. Writing a value of '0' to this file disables the

feature. Writing a value of '-1' to this file resets the value to the

default setting.

throttle_sample_time (RW)

-------------------------

This is the time window that blk-throttle samples data, in millisecond.

blk-throttle makes decision based on the samplings. Lower time means cgroups

have more smooth throughput, but higher CPU overhead. This exists only when

CONFIG_BLK_DEV_THROTTLING_LOW is enabled.

write_zeroes_max_bytes (RO)

---------------------------

For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of

bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES

is not supported.

zone_append_max_bytes (RO)

--------------------------

This is the maximum number of bytes that can be written to a sequential

zone of a zoned block device using a zone append write operation

(REQ_OP_ZONE_APPEND). This value is always 0 for regular block devices.

zoned (RO)

----------

This indicates if the device is a zoned block device and the zone model of the

device if it is indeed zoned. The possible values indicated by zoned are

"none" for regular block devices and "host-aware" or "host-managed" for zoned

block devices. The characteristics of host-aware and host-managed zoned block

devices are described in the ZBC (Zoned Block Commands) and ZAC

(Zoned Device ATA Command Set) standards. These standards also define the

"drive-managed" zone model. However, since drive-managed zoned block devices

do not support zone commands, they will be treated as regular block devices

and zoned will report "none".

zone_write_granularity (RO)

---------------------------

This indicates the alignment constraint, in bytes, for write operations in

sequential zones of zoned block devices (devices with a zoned attributed

that reports "host-managed" or "host-aware"). This value is always 0 for

regular block devices.

independent_access_ranges (RO)

------------------------------

The presence of this sub-directory of the /sys/block/xxx/queue/ directory

indicates that the device is capable of executing requests targeting

different sector ranges in parallel. For instance, single LUN multi-actuator

hard-disks will have an independent_access_ranges directory if the device

correctly advertizes the sector ranges of its actuators.

The independent_access_ranges directory contains one directory per access

range, with each range described using the sector (RO) attribute file to

indicate the first sector of the range and the nr_sectors (RO) attribute file

to indicate the total number of sectors in the range starting from the first

sector of the range.  For example, a dual-actuator hard-disk will have the

following independent_access_ranges entries.::

        $ tree /sys/block/<device>/queue/independent_access_ranges/

        /sys/block/<device>/queue/independent_access_ranges/

        |-- 0

        |   |-- nr_sectors

        |   `-- sector

        `-- 1

            |-- nr_sectors

            `-- sector

The sector and nr_sectors attributes use 512B sector unit, regardless of

the actual block size of the device. Independent access ranges do not

overlap and include all sectors within the device capacity. The access

ranges are numbered in increasing order of the range start sector,

that is, the sector attribute of range 0 always has the value 0.

Jens Axboe <jens.axboe@oracle.com>, February 2009